In order to manufacture semiconductor products such as a semiconductor integrated circuit and the like, it is general that a semiconductor wafer is repeatedly subjected to various kinds of processes, including film-forming, etching, oxidizing and diffusing, ashing and reforming. As the semiconductor products become highly dense and miniaturized, the various kinds of processes need to be performed with high in-plane uniformity in view of the improvement of a product yield rate.
In this regard, a plasma processing apparatus will be described as an example of the conventional single wafer plasma processing apparatus. Plasma processing apparatuses are disclosed in, e.g., Japanese Patent Laid-open Publication Nos. H3-191073, H5-343334, H9-181052 and 2002-311892. FIG. 11 is a schematic configuration view showing a conventional plasma processing apparatus.
Referring to FIG. 11, the plasma processing apparatus 2 includes an evacuable processing chamber 4 and a mounting table 6, provided within the processing chamber 4, for mounting a semiconductor wafer W thereon. The mounting table 6 is supported by an “L”-like support arm 7 extending from a sidewall of the processing chamber 4. A disk-like ceiling plate 8 that transmits microwaves is air-tightly provided in a ceiling portion of the processing chamber 4 in a facing relationship with the mounting table 6. The ceiling plate 8 is made of aluminum nitride, quartz or the like. A gas nozzle 9 for introducing a specified gas into the processing chamber 4 is formed in the sidewall of the processing chamber 4.
On an upper surface of the ceiling plate 8, there are installed a disk-like planar antenna member 10 having a thickness of about several millimeters and a retardation member 12 for shortening the wavelength of microwaves in a radial direction of the planar antenna member 10. The retardation member 12 is made of, e.g., a dielectric material. A plurality of microwave irradiation holes, e.g., elongated through-holes, is formed in the planar antenna member 10. The microwave irradiation holes 14 are usually arranged in a concentric pattern or a spiral pattern. A central conductor 18 of a coaxial waveguide tube 16 is connected to a central portion of the planar antenna member 10 so that the microwaves of, e.g., 2.45 GHz, generated in a microwave generator 20 is converted to a specified vibration mode by means of a mode transducer 22 and then sent to the planar antenna member 31. While propagating radially of the planar antenna member 10, the microwaves are irradiated through the microwave irradiation holes 14 formed in the planar antenna member 1. Then, the microwaves are transmitted through the ceiling plate 8 and introduced into the processing chamber 4, whereby the microwaves generate plasma in a processing space S within the processing chamber 4.
A gas exhaust port 24 is formed in a central region of a bottom portion 4A of the processing chamber 4. A pressure control valve 26 is attached to the gas exhaust port 24. The pressure within the processing chamber 4 is regulated by controlling the opening degree of the pressure control valve 26 and consequently changing the area of an aperture of a valve port 28. The pressure control valve 26 is of, e.g., a gate valve. The opening degree, i.e., the aperture area, of the pressure control valve 26 is controlled by slidingly moving a valve body 30 in the direction indicated by arrow 31, e.g., in a horizontal direction. A turbo molecular pump 32 as a vacuum pump of a gas exhaust system is connected to a gas outlet side of the pressure control valve 26 so that it can evacuate the processing chamber 4 into a vacuum state. With this configuration, plasma is generated in the processing space S within the processing chamber 4 and a semiconductor wafer W is subjected to plasma processes such as plasma etching, plasma film-forming and the like.
In order to increase the yield rate of products as mentioned above, the plasma processes need to be uniformly performed over a wafer plane. In a general single wafer processing apparatus as well as the plasma processing apparatus noted above, the processing uniformity is heavily affected by a way of causing a gas to flow within the processing space S. To that end, the gas exhaust port 24 and the valve port 28 of the pressure control valve 26 are arranged in the central region of the bottom portion 4A of the processing chamber 4 while, for example, a center axis of the mounting table 6 is in alignment with the center axis of the gas exhaust port 24 and the valve port 28. In this way, the ambient gas within the processing space S is allowed to uniformly flow around the mounting table 6 and then flow down toward the gas exhaust port 24 positioned below the mounting table 6.
With the arrangement of the pressure control valve 26 as set forth above, if the valve opening degree is equal to 100%, the ambient gas within the processing space S flows down in a state that it is uniformly distributed around the mounting table 6. However, if the valve opening degree is small as is the case in an actual process, there occurs a deviation in the gas flow. In other words, when the afore-mentioned pressure control valve 26 is used for the purpose of pressure control, it is necessary to regulate the target control pressure as the process pressure with a good controllability with respect to a pressure range around the target control pressure. To this end, it is general that the range of the valve opening degree practically used in the process is set equal to about 5 to 40%. The practical use range of the valve opening degree noted above is also recommended by control valve makers as a desirable use range.
Stating differently, if the processing apparatus is designed to control the process pressure with an excessively small valve opening degree or an excessively high valve opening degree, the change in gas exhaust conductance relative to the change in the valve opening degree is too small or too great to stably control the pressure. In view of this, the processing apparatus available in practice is as a whole designed to ensure that the pressure range around the target control pressure can be stably controlled in a process by using the pressure control valve 26 at a valve opening degree of about 20%.
However, in case the valve opening degree of the pressure control valve 26 is as small as 20%, the valve port 28 is mostly closed by the valve body 30 as illustrated in FIG. 12, which is a plan view of the valve port 28. The opening region M (the hatched region in FIG. 12) through which a gas actually passes has a shape of crescent. Therefore, the opening region M is positioned far away from the center axis 6A of the mounting table 6.
This creates a deviation in the flow rate of a gas flowing down around the mounting table 6 as indicated by an arrow 34 in FIG. 11. As a consequence, it becomes impossible to uniformly exhaust the gas from around the mounting table 6. This poses a problem in that the in-plane uniformity is reduced when processing a wafer. This problem becomes conspicuous as the size of a gas exhaust port grows larger together with the increase in the wafer size to 300 mm and the resultant increase in the size of the processing chamber 4.